This invention relates generally to radiological well logging methods and apparatus for investigating the subsurface earth formations traversed by a borehole and, more specifically, relates to an improved system for pulsed neutron gamma ray logging methods and apparatus wherein gamma rays resulting from neutron inelastic scattering and thermal neutron capture are selectively detected and the spectral distribution of the gamma rays is determined in conjunction with the stripping of downscattered high energy gamma rays.
The selective detection of characteristic gamma rays emanating from earth elements undergoing neutron bombardment is appreciated by those skilled in the art as a method for identifying such elements. More specifically, the detection of gamma rays from carbon, oxygen, silicon, calcium and certain other elements enables the identification of the general rock types in formations traversed by boreholes and the determination of the presence or absence of hydrocarbons within their pore spaces. To identify these elements, both high and low energy reactions must be detected; thus, the neutron source must be pulsed and measurements made during the neutron burst when high energy reactions occur and between bursts when thermal capture reactions occur. Hence, the detector and neutron source must be synchronized.
In well logging applications, the neutron source and detector in the subsurface instrument are connected to the surface analyzing and recording equipment by 20,000 or more feet of cable which carries power, control and detector signals. Since the amplitude of the detector pulses varies in proportion to the energy of the detected gamma rays, the logging cable must not significantly degrade the energy resolution of the system. The seven-conductor cables which are widely used in the well logging industry have been found to be generally acceptable for gamma ray spectral analysis despite their poor high frequency response. The detector and sync pulses applied to the surface end of the logging cable are widened during their transit over the line and are several microseconds wide when they reach the surface end of the cable. Typically, a unipolar pulse which is two to three microseconds wide at the subsurface end of a 20,000 foot cable will be 10 to 12 microseconds wide at the surface end. While this time spreading is of little significance at low source pulsing frequencies, it does place an upper limit on the usable source pulsing frequency if a synchronization pulse is transmitted each time the source is pulsed. Furthermore, it should be appreciated that a high pulsing frequency is desirable for the inelastic detection systems in order to obtain the counting rates appropriate for good statistical accuracy. Since the pulses are spread in time by their transit over the logging cable, there is a greater probability of pulse pile-up on the line than in the gamma ray detector itself. In order to eliminate detector pulse pile-up on the line, it has already been found desirable to incorporate the circuit described in U.S. Pat. No. 3,739,172, assigned to the assignee of this application, which allows only one pulse per gate interval to be fed to the cable for transmission to the surface. Such a circuit allows a pulse which occurs as late as 100 nanoseconds before the end of the gating interval to be transmitted as a full width pulse.
By example, if a 10 microsecond wide neutron burst and a 10 microsecond wide inelastic detector gate are used with a 10 microsecond wide capture gate in a system pulsed at 20 KHz, there is little time left in the repetition period for a sync pulse to be transmitted. This is because the detector pulses from a particular detector gate fall within a 20 to 22 microsecond wide interval at the receiving end of the cable. This time is the sum of the 10 microsecond wide detector gate and the 10 to 12 microsecond wide pulses received at the surface since the pulse may well occur right at the end of the detector gate interval. With lines shorter than 20,000 feet, the time spreading is correspondingly less.
To use these neutron bursts and detector gate widths and allow a small safety margin against coincidence, it would be necessary to increase the pulsing and detection period to about 60 microseconds. Since one of the problems with inelastic gamma ray well logging systems is that of obtaining sufficient counting rates to produce a statistically accurate measurement, it is desirable to operate at the higher pulsing frequency, for example, 20 KHz, having a pulsing and detection period of 50 microseconds.
High energy gamma rays, incident upon a NaI (Tl) scintillation counter, can be downscattered in the crystal and counted as a lower energy pulse. In the case of systems for determining a carbon-oxygen ratio, gamma rays of energy levels higher than oxygen can be degraded and appear in the energy interval used to measure both the oxygen and the carbon gamma rays. Therefore, it has been determined in accordance with the present invention that subtracting or "stripping" all or a portion of such high energy gamma rays from the gamma rays recorded in the carbon and oxygen intervals will improve the sensitivity to oil in the carbon/oxygen logging instrument.
It is therefore the primary object of this invention to provide new and improved method and apparatus for reducing the effect of downscattered gamma rays upon spectral analysis well logging systems;
It is also an object of the present invention to provide new and improved method and apparatus for increasing the carbon/oxygen sensitivity between water-filled and oil-filled porosity measurements in well logging systems.
The objects of the invention are accomplished, generally, by method and apparatus which subtract at least a portion of the downscattered high energy gamma rays from the spectrum of interest being analyzed in a well logging system.